It only takes one hit/sack on Tannehill to end his season and Miami's hopes of making the playoffs. You want your best pass blocking Tackle protecting your franchise QB's blindside.
This guy protected Lucks blindside for 3 years then was drafted and told to play RT (I believe a position he never played) and did a decent job (not great) for a rookie then Long gets hurt and he's asked to jump over to LT again. This is not asking somebody to switch from the fries to the register at McDonalds people, it was very difficult what he did last year.
I think our FO believes he can play the position but doesn't want to count on Martin alone. Let's give Martin some competition and battle it out in training camp....the best man plays LT.